Niobium-base alloys

ABSTRACT

Niobium-base alloys containing tungsten, hafnium and carbon are improved in respect of their strength in the range 300*-1,000* C. by the addition of silicon and by restriction of the atomic ratio of hafnium to carbon to certain critical limits. At least 100 parts per million of silicon is necessary to produce an upward trend in the ultimate strength of the alloy.

United States Patent Kelcher Feb. 1 11972 [54] NIOBIUM-BASE ALLOYS [56] References Cited [72] Inventor: Robert William Kelcher, Sutton Coldfield. FOREIGN PATENTS OR APPLICATIONS 889,099 2/1962 Great Britain ..,75/ 174 [73] Assignee: Imperial Metal Industries (Kynoch) Limited, Birmingham England 1,013,220 12/1965 Great Britain ..75/174 [22] Filed: 7 Dec. 9, 1969 Primary Examiner-Charles N. Lovell [21] pp No; Attorney-Cushman, Darby & Cushman ABSTRACT [30] Fol-mg Application Pnomy Data Niobium-base alloys containing tungsten, hafnium and carbon Dec. 14, 1968 Great Britain ..60,428/68 are improved in respect of their strength in the range 300-1,000 C. by the addition of silicon and by restriction of [52] US. Cl ..148/ 12.7, 75/174, 148/325 the ato ic ratio of hafnium to carbon to certain critical limits. l f C226 27/00 At least 100 parts per million of silicon is necessary to produce Fleld Of Search 32, an upward trend in (he ultimatg strength ,ofthe alloy 7 Claims, 4 Drawing Figures Ton/in l'i bars .70 5 S- 45 23 ppm Y 40 230 PP'" 295 ppm Si Stress I I 20 ppm I I20 ppm s:

/q 20 ppm 31' 3 1 TYP UTS (T Tensometer' Yield Point NlOBIUM-BASE ALLOYS This invention relates to niobium-base alloys.

BACKGROUND OF THE INVENTION Niobium-base alloys possess good high-temperature strength and in particular alloys containing tungsten up to about 30 percent, hafnium up to about 4 percent and carbon up to about 0.2 percent and sometimes also containing molybdenum and zirconium have good stress-rupture life, tensile properties and ductility.

The proof and ultimate tensile strengths of niobium-tungsten-hafnium-carbon alloys, however, decrease rapidly between room temperature and 400 C. and then remain approximately constant to abut l,000 C. This low intermediate temperature strength is a disadvantage in certain applications, e.g., turbine blades in which the most highly stressed part (the blade root) operates at about 600 C.

We have found the strength of such alloys in the 3001,000 C. temperature range can be markedly improved by a modification of the composition combined with certain critical limits to the atomic ratio of hafnium to carbon and a certain heattreatment sequence.

SUMMARY OF THE INVENTION According to the present invention, a niobium'base alloy possessing a high-yield point and tensile strength at elevated temperature consists of -30 percent tungsten, 0.06-0.14 percent carbon, hafnium in an atomic ration of I.2-2.2:1 carbon, 0.0I0.l percent silicon and optionally up to 6 percent molybdenum replacing an equivalent atomic percentage of tungsten; and optionally up to 4 percent zirconium, balance niobium, and impurities.

DESCRIPTION OF THE PREFERRED EMBODIMENT Such alloys show a considerable improvement in tensile strength in the range 400-l,000 C. and in yield point over previously known alloys of this type without silicon. This improvement is achieved without detriment to the ductility or to the stress rupture life of the alloy, and is limited to a narrow range of silicon content. With a silicon content of below about I00 p.p.m., tensile strength and yield point at elevated temperature are fairly constant. Above 120 p.p.m. of silicon, these properties increase markedly in value while the excellent ductility and stress rupture life remain constant. when a silicon content of about 500 p.p.m. is reached, there is a decrease in ductility as the known embrittling tendency of silicon becomes effective.

The improvement in strength is believed to be due to the presence of a precipitate of silicon carbide and there are three requirements for the formation of a suitable precipitate, as follows:

a. There must be sufficient silicon present, which in this context means more than 100 p.p.m.

b. There must be sufiicient carbon (present as metastable Nb C needles or NbC plates) to react with that silicon. The amount of carbon available depends upon the ratio of hafnium to carbon contents and on the solution treatment temperature. If the ratio of number of hafnium atoms to number of carbon atoms exceeds about 2:1 there will be little metastable carbide after solution treatment, for the carbon will all be firmly locked up in (Nbl-If)C carbide. As the hafniumzcarbon ratio decreases below 2:1 the amount of metastable carbide increases, and at constant hafniumzcarbon ratio the amount of metastable carbide increases with increasing solution treatment temperature.

c. The third requirement for the formation of an effective dispersion of silicon carbide is a network of dislocations for the silicon carbide to nucleate on. If few dislocations are present then nucleation of silicon carbide is difficult because of its complex structure, and a small number of coarse particles form with little effect on strength.

The first step in the development of optimum properties is a solution treatment. This dissolves the silicon and some of the carbon; during cooling the carbon which was in solution, precipitates out in ametastable form, either as Nb C needles or NbC plates. In the second step the material is worked to generate dislocations within the metal. Finally during aging, a fine precipitate forms on the dislocations, and it is this precipitate which is responsible for the strengthening effect by hindering the movement of the dislocations.

The precipitate particles are very small (about A. diameter) and have not been positively identified, but are believed to be silicon carbide rather than hafnium carbide, as hafnium carbide particles 120 A. in diameter would be coherent with the matrix and show strain fields when observed by transmission electron microscopy, whereas no strain fields have been observed. Silicon carbide would not be expected to show strain fields as its structure is very different from the simple body centered cubic structure of niobium and for these reasons it is assumed that the precipitate is silicon carbide.

Silicon has little effect below 100 p.p.m. and alloys will be too brittle if they contain more than 500 p.p.m. with the optimum hafniumzcarbon atomic ratio. However, with higher hafnium:carbon ratio, the silicon content can exceed 500 p.p.m. without causing brittleness and the silicon contents over which the benefits are obtained are I00-L000 p.p.m., the preferred range being -500 p.p.m.

The maximum hafniumacarbon atomic ratiowhich can be tolerated is 22:1 and the minimum is 1.2:l but preferably the range lies within these limits, the hafnium being l.52.0:l carbon.

The solution treatment temperature depends upon the hafniumzcarbon ratio of the alloy and the tungsten content and increases as these parameters increase. The minimum temperature for an alloy having a nominal composition of niobium, 17 percent tungsten, 3.5 percent hafnium, 0.12 percent carbon (SU 3 l is 1,600 C. and the maximum, which is determined by the onset of brittleness caused by the presence of excessive carbide at the grain boundaries is about 1,750 C. Preferably the alloy is heated at these temperatures for about 1 hour.

Working is applied to the alloy while in the solution-treated condition and should be between 10 percent and 50 percent reduction at a working temperature of l00800 C. The preferred amount of working is 15-25 percent at 200-800 C. for Su 31.

After working, for example to effect final shaping as in a turbine blade, the alloy is aged between 950-l,l50 C., preferably l,lO0 C. for 5 hours. Below 950 C., the formation of silicon carbide is too slow and above 1,150 C. the precipitate overages.

BRIEF DESCRIPTION OF THE DRAWINGS The influence of silicon content and hafniumzcarbon atomic ratio of the ultimate tensile strength of the alloy SU 31, referred to above, is shown in the Table The samples were heat-treated. Work hardening of the alloy in the solutiontreated condition in relation to silicon content is illustrated in FIG. 1 and ultimate tensile strength as a function of silicon content is shown in FIG. 2.

The general efiect of increasing silicon content is to raise the proof and ultimate strengths of the alloy by about the same extent for all levels within the range of silicon content, as shown by the slope of the lines in FIG. 1. When the heat-treatment and working sequence is kept constant at 1 hour at 1,600 C. solution treatment, 15 percent reduction and again at l,l00 C. for 5 hours, the ultimate tensile strength varies with the silicon content and with the hafnium-carbon ratio as shown in the Table. Sample 6 of the Table shows for comparison purposes an alloy having a hafniumwarbon ratio in excess of the maximum specified in the present invention and it will be noted that there is a large decrease in ultimate strength compared with sample 5, although the silicon content is greater than in sample 5.

lam W"- F 10. 2 shows the effect of silicon content on ultimate tensile strength in graphical form based on the tests reported in the Table. From the curve it will be seen that below about 120 p.p.m. silicon has little effect but above 120 p.p.m. its effect increases rapidly. Those samples having hafniumzcarbon ratios below 2:1 fall very close to the curve, but the single result for the sample having a hafniumzcarbon ratio of 2.3:1 is located well away from the curve, thus indicating the criticality of the hafniumzcarbon ratio. At constant silicon content, alloys with higher hafnium:carbon ratios are weakercompare samples 1 and 2 in the Table-and at a very high hafniumzcarbon ratio of 2.3:1 (sample 6) the strengthening effect of silicon is nullified because no carbon is available for reaction with the silicon to form silicon carbide.

The ultimate tensile strength can be raised by increasing the solution treatment temperature. For example, sample increased its strength from 69.4 to 74 hectobars (hbars (45.1 to 48 tonf/in?) when the solution treatment temperature was increased from 1,600 to 1,700 C. all other parameters remaining constant.

The gain in strength by the addition of silicon is not accompanied by a severe drop in ductility or in stress rupture life. FIG. 3 shows the effect of silicon on elongation and reduction in area values and FIG. 4 shows the influence of silicon on the stress rupture properties of the alloy SU 31. The alloys were in the heat-treated, worked and aged condition.

Tungsten may be varied within the range specified and the effect of the variations is that the strength levels are raised or lowered in proportion to the amount of tungsten present.

1 claim:

tensile strength at elevated temperature consisting of 10-30 percent tungsten, 0.06-0.14 percent carbon, hafnium in an atomic ratio of 1.2-2.2:1 carbon, 0.01-0.l percent silicon and optionally up to 6 molybdenum replacing an equivalent atomic percentage of tungsten; and optionally up to 4 percent zirconium, balance niobium and impurities, said alloy exhibiting a microstructure wherein the silicon is present as silicon carbide precipitated on a network of dislocations within the alloy.

2. An alloy as claimed in claim 1 in which the hafniumzcarbon atomic ratio is in the range 1.5-2.0 hafnium to 1 carbon.

3. An alloy asclaimed in claim 1 in which the silicon content is 0.015-0.05 percent.

4. An alloy as claimed in claim 1 in which the tungsten content is 17 percent, hafnium content is 3.5 percent and carbon content is 0.12 percent.

5. A method for producing a niobium-base alloy possessing a high-yield point and tensile strength at elevated temperature comprising: heating an alloy consisting of 10-30 percent tungsten, 0.06-0.14 percent carbon, hafnium in an atomic ratio of 1.2-2.2:1 carbon, 0.01-0.1 percent silicon, up to 6 percent molybdenum replacing an equivalent atomic percentage tungsten, and up to 4 percent zirconium, balance niobium and impurities to a temperature in the range 1,600 to 1,7S0 C., to dissolve the silicon and carbon; cooling to precipitate carbon in metastable form; working said alloy to effect a reduction of 10 percent to 50 percent at a temperature of to 800 C.

to generate dislocations within the metal; and ageing by heating to the temperature range of 950 to 1,1 50 C. to produce a fine precipitate of silicon carbide on the dislocations.

6. The method of claim 5 in which aging is effected by heating at 1,100 C. for 5 hours.

7. The method of claim 5 in which working is carried out to effect a reduction of 15-25 percent at a temperature between 200-800 C. 

2. An alloy as claimed in claim 1 in which the hafnium:carbon atomic ratio is in the range 1.5-2.0 hafnium to 1 carbon.
 3. An alloy as claimed in claim 1 in which the silicon content is 0.015-0.05 percent.
 4. An alloy as claimed in claim 1 in which the tungsten content is 17 percent, hafnium content is 3.5 percent and carbon content is 0.12 percent.
 5. A method for producing a niobium-base alloy possessing a high-yield point and tensile strength at elevated temperature comprising: heating an alloy consisting of 10-30 percent tungsten, 0.06-0.14 percent carbon, hafnium in an atomic ratio of 1.2-2.2:1 carbon, 0.01-0.1 percent silicon, up to 6 percent molybdenum replacing an equivalent atomic percentage tungsten, and up to 4 percent zirconium, balance niobium and impurities to a temperature in the range 1,600* to 1,750* C., to dissolve the silicon and carbon; cooling to precipitate carbon in metastable form; working said alloy to effect a reduction of 10 percent to 50 percent at a temperature of 100* to 800* C. to generate dislocations within the metal; and ageing by heating to the temperature range of 950* to 1,150* C. to produce a fine precipitate of silicon carbide on the dislocations.
 6. The method of claim 5 in which aging is effected by heating at 1,100* C. for 5 hours.
 7. The method of claim 5 in which working is carried out to effect a reduction of 15-25 percent at a temperature between 200*-800* C. 